A semiconductor photodetector generates a free-carrier pair (electron-hole) when it absorbs a photon. When the photodetector is subjected to an electric field (by the application of a bias voltage to the photodetector), free-carriers generated in the photodetector give rise to a macroscopic electric current.
A useful photodetector is characterized by high overall efficiency and high sensitivity. Efficiency can be defined as the number of free carriers that are generated per incident photon. Consequently, high efficiency implies a high generated current for a given incident optical power. Sensitivity is characterized by the minimum optical signal that gives rise to a current that can be distinguished from the background current due to noise (e.g., dark current, thermal noise, Johnson noise, 1/F noise, etc.).
One widely-used type of photodetector is the avalanche photodetector. Avalanche photodetectors have high sensitivity and, in fact, can be made sensitive enough to detect even a single photon. Avalanche photodetectors are so named because of the “avalanche” of free-carrier pairs that is generated by the detector. The “avalanche” is the result of a multiplication of the free-carrier pairs, The multiplication occurs when the free-carrier pairs that were generated by incident photons are accelerated to high energies by an applied reverse bias voltage. As the accelerated free carriers travel through the multiplication region of the avalanche photodetector, they collide with bound carriers in the atomic lattice of the multiplication region, generating more free carriers through a process called “impact ionization.”
The current flow in the avalanche photodetector is directly related to the number of free carriers generated from electron-hole pairs. The gain of a photodetector (i.e., the increase in the number of free carrier pairs) is a function of the reverse bias voltage applied to the photodetector.
An avalanche photodetector is characterized by a “breakdown voltage.” When the avalanche photodetector is biased above its breakdown voltage, carrier generation can become self-sustaining and result in run-away avalanche. In order to function as a single-photon detector, an avalanche photodetector is biased above its breakdown voltage. This is referred to as “arming” the avalanche photodetector. Once the detector is armed, the single free carrier created by the absorption of a single photon can create a runaway avalanche resulting in an easily detectable macroscopic current. It is also possible for a free carrier to be created by mechanisms other than photon absorption (e.g., thermal excitation and carrier tunneling). These “dark” carriers can give rise to the same easily detectable macroscopic current, in this case referred to as false counts, or “dark counts.” Dark counts constitute noise in a single-photon avalanche detector, and therefore reduce its sensitivity.
After a photon (or dark count) is detected, it is necessary to stop the self-sustained avalanche in order to make further use of the detector. In order to halt the avalanche process, the bias voltage of the avalanche photodetector is reduced below its breakdown voltage. This process is referred to as “quenching” the avalanche photodetector. Although quenching stops the avalanche process, not all free carriers are swept out of the avalanche region. Instead, some carriers become trapped in trap states that exist in the multiplication region due to crystalline defects or other causes which create energy levels within the semiconductor band gap of the multiplication region material.
At a later time, trapped carriers “detrap,” again becoming free carriers. These detrapped carriers can become an additional source of dark counts. The creation of additional dark counts caused by spurious, uncontrolled emission of trapped charges after quenching is referred to as “afterpulsing.” Afterpulsing raises the total dark count rate above the baseline dark count rate established by thermal carrier emission and carrier tunneling in the absence of afterpulsing. Since any increase of dark count rate degrades the performance of a single-photon detector, elimination of afterpulsing is of great interest.
Several strategies exist in the prior art for reducing afterpulsing. Trapped charges will generally become free carriers in random fashion due to their thermal emission from the trap. Therefore, one approach used is to simply wait a sufficient period of time after quenching to allow trapped charges to detrap on their own (i.e., the inherent “detrapping time.” If the inherent detrapping time is long, this approach leads to an undesirably long period of time when the single-photon detector is inoperable. In gated-mode operation, wherein a bias voltage pulse is periodically applied to arm the avalanche photodetector (i.e., a “gating pulse”), simply waiting for thermal emission of trapped carriers reduces the repetition rate at which single photons can be measured.
A second prior-art approach for reducing afterpulsing is to operate the single-photon detector at an elevated temperature to promote detrapping. But operating at an elevated temperature results in an increase in the baseline dark count rate due to an increase in thermal carrier emission and carrier tunneling processes.
In a third prior-art approach for reducing afterpulsing, trapped carriers are photoionized via “sub-band illumination.” In this approach, the photodetector is illuminated by a beam of light. The energy of the photons in this beam of light is a function of the wavelength of the light. Photons in longer-wavelength light have relatively lower energy than photons in shorter-wavelength light. The wavelength of light used in this prior-art approach is selected to provide photoionization energy sufficient only to detrap carriers, but insufficient to liberate carriers that are not in trapped states.
To avoid the detection of the sub-band illumination by the photodetector, the wavelength of light used for sub-band illumination must be longer than the detection limit, or “cutoff wavelength,” of the absorbing material in the photodetector. In the case of an indium-phosphide-based avalanche photodetector with an indium-gallium-arsenide absorbing material, the wavelength of light used for sub-band illumination is greater than 1700 nanometers.
There exists a need, therefore, for a single-photon detector with reduced afterpulsing that overcomes some of the limitations of the prior art.